The present invention relates to silicon nitride bonded magnesia brick particularly suited for use in the processing of molten aluminum and its alloys, particularly aluminum-lithium alloys which are used in aerospace applications. These alloys typically contain about 2-3 wt.% lithium which significantly increases the strength of aluminum and decreases weight.
A number of refractory compositions have been proposed for the processing of such molten aluminum materials. However, they are not entirely satisfactory, particularly in the processing of molten aluminum-lithium alloys which have a very corrosive and fluid character. These alloys have a very sharp melting point and above that point have a viscosity like that of water. Also, superheating of the alloys is necessary to prevent freezing of the metal before shape forming can be accomplished. At such superheating temperatures lithium tends to vaporize out of the alloy and because of its low surface tension tends to penetrate into most refractory linings. Because of lithium's corrosive nature most conventional refractories are readily destroyed. An additional deficiency with most refractories is that after reaction with such alloys the eroded refractory contaminates such alloys resulting in a severe loss in the strength of the metal. Common contaminants such as iron, silicon, calcium, sodium, and the like must not, in total, exceed 0.05 wt.% in these alloys in order not to adversely affect the strength.
For these reasons, only two refractories have been found which can provide a reasonable containment of these alloys. These are an oxide bonded magnesia and a silicon nitride bonded silicon carbide. However, even these are not entirely satisfactory. Both contaminate the alloy to an unacceptable degree. The oxide bonded magnesia refractories are also deeply penetrated by the molten alloys and the silicon nitride bonded refractories, while not deeply penetrated, are adversely weakened by attack on the silicon carbide grains.
In addition to these deficiencies processing of mixes to make the shapes; such as brick, by molding were not entirely satisfactory in that the brick showed weakness at the press in the formation of "pressure" cracks. Efforts to make a shape free of cracking even after many mix changes have not been satisfactory.